Combinatorial analysis of calcium-binding proteins in larval and adult zebrafish primary olfactory system identifies differential olfactory bulb glomerular projection fields.

In the zebrafish (Danio rerio) olfactory epithelium, the calcium-binding proteins (CBPs) calretinin and S100/S100-like protein are mainly expressed in ciliated or crypt olfactory sensory neurons (OSNs), respectively. In contrast parvalbumin and calbindin1 have not been investigated. We present a combinatorial immunohistological analysis of all four CBPs, including their expression in OSNs and their axonal projections to the olfactory bulb in larval and adult zebrafish. A major expression of calretinin and S100 in ciliated and crypt cells, respectively, with some expression of S100 in microvillous cells is confirmed. Parvalbumin and calbindin1 are strongly expressed in ciliated and microvillous cells, but not in crypt cells. Moreover, detailed combinatorial double-label experiments indicate that there are eight subpopulations of zebrafish OSNs: S100-positive crypt cells (negative for all other three CBPs), parvalbumin only, S100 and parvalbumin, parvalbumin and calbindin1, and parvalbumin and calbindin1 and calretinin-positive microvillous OSNs, as well as a major parvalbumin and calbindin1 and calretinin, and minor parvalbumin and calbindin1 and calretinin-only-positive ciliated OSN populations. CBP-positive projections to olfactory bulb are consistent with previous reports of ciliated OSNs projecting to dorsal and ventromedial glomerular fields and microvillous OSNs to ventrolateral glomerular fields. We newly describe parvalbumin-positive fibers to the mediodorsal field which is calretinin free, with its anterior part showing additionally calbindin1-positive fibers, but absence thereof in the posterior part, indicating an origin from microvillous OSNs in both parts. One singular glomerulus (mdG2) exhibits S100 and parvalbumin-positive fibers, apparently originating from all crypt cells plus some microvillous OSNs. Arguments for various olfactory labeled lines are discussed.

Correlated basal expression of immediate early gene egr1 and tyrosine hydroxylase in zebrafish brain and downregulation in olfactory bulb after transitory olfactory deprivation.

Imprinting on kin occurs during the sixth day of larval development in zebrafish and depends on olfactory signals. In rodents, the immediate early gene egr1 is involved in maintaining the dopaminergic phenotype of periglomerular olfactory bulb cells in an activity dependent way. Furthermore, egr1 is upregulated in medial amygdalar dopamine cells in some rodents (prairie voles) dependent on social pheromone interactions. Thus, we aimed to investigate whether egr1 is involved in imprinting processes and later kin recognition in zebrafish in olfactory centers, such as the olfactory bulb and suspected medial amygdala. In the present paper, we focus on a basic investigation of basal egr1 expression throughout zebrafish brain development and its co-localization with tyrosine hydroxylase as a marker for dopaminergic neurons. Indeed, there is unambiguous co-localization of egr1 and tyrosine hydroxylase in the zebrafish olfactory bulb and hypothetical medial amygdala. Furthermore, as in rodents, ipsilateral transient olfactory deprivation through Triton X-100 treatment of the olfactory epithelium leads to downregulation of egr1 and tyrosine hydroxylase expression in the olfactory bulb, but apparently not in secondary olfactory targets of the zebrafish brain. This indicates that similar processes might be at work in zebrafish and rodent olfactory systems, but their more specific involvement in imprinting in zebrafish has to be further tested.